The invention relates to a hydraulic control arrangement which is used for a mobile working machine, especially for a wheel loader.
It is known from DE 39 09 205 C1 to make use for the purpose of damping the pitching vibrations of wheel loaders which occur, especially, with a full loading shovel and at a high driving speed of a damping system which is a component of the hydraulic control arrangement of the wheel loader. For the purpose of vibration damping, the generally two hydraulic lifting cylinders for raising and lowering the loading shovels can be connected via a shutoff valve to a hydraulic accumulator which can be charged by a hydraulic pump via a fill line which branches off from the pump line upstream of the directional control valve block. The shutoff valve, which is arranged between the hydraulic accumulator and the lifting cylinders, is closed as long as the loading shovels are working, and can be opened by the driver or automatically as soon as pitching vibrations occur during driving, or as soon as the driving speed is above a specific value, for example above 6 km/h.
As a consequence of the fact that the filling line branches off upstream of the directional valve control block, the hydraulic accumulator is charged not only upon actuation of the directional control valve assigned to the lifting cylinders, but upon actuation of any direction control valve which leads to a build up of pressure in the pump line. For example, the actuation of a steering valve belonging to a hydraulic steering system of the working machine can also lead to an inflow of pressure fluid to the hydraulic accumulator. If the shutoff valve is then also open, an uncontrolled movement of the lifting cylinders can take place.
Another damping system against pitching vibrations, which is likewise part of the hydraulic control arrangement of a working machine, is known from DE 41 29 509 C2. In this case, the filling line branches off from a working line which runs between the lifting cylinders and the directional control valve assigned thereto. The shutoff valve arranged in the filling line is pressure-controlled and can be opened by the load pressure, prevailing in the working line, of the lifting cylinders against the accumulator pressure, which can be applied to a rear control chamber on the valve element of the shutoff valve, and against the force of a weak compression spring. The accumulator pressure is thus in each case only slightly lower than the highest load pressure of the lifting cylinders which occurs during a working cycle. In order to damp the pitching vibrations, the rear control member of the shutoff valve is unloaded via a pilot valve to the tank, with the result that the shutoff valve can be opened and pressure fluid can be pushed back and forth freely between the hydraulic accumulator and the lifting cylinders.
It has emerged that because of special kinematic relationships on a wheel loader when the shovel is being pushed into the earth, a higher pressure can be built up in the lifting cylinders than prevails in the hydraulic accumulator, and then pressure fluid is displaced into the hydraulic accumulator from the lifting cylinders even when the assigned directional control valve is not actuated. The wheel loader is raised at the front, with the result that the front wheels turn in the air and the rear wheels dig in. This is disadvantageous for the loading operation.